1. Field of the Invention
The present invention relates to an automatic wheel alignment measurement system and, in particular, an improved system for making toe measurements.
2. Description of the Prior Art
Proper alignment of the wheels of an automotive vehicle is important both for proper handling of the vehicle, as well as proper tire wear. The wheel alignment parameters which are measured and adjusted in order to achieve proper wheel alignment are camber, caster, steering axis inclination (SAI) and toe.
Camber is defined as the angle which the wheel makes with respect to a vertical plane when looking at the wheel from either the front or the rear of the automobile. Any wheel of an automobile can have camber.
Caster is an angle which the steering axis of a steerable wheel (typically one of the front wheels makes with respect to the vertical plane which is perpendicular to the longitudinal direction of the automobile.
Steering axis inclination (SAI) is the inclination of the steering axis with respect to a vertical plane which is parallel to the longitudinal direction of the automobile.
Toe was originally defined as the difference of the distance between the front edges of two opposite wheels and the distance between the rear edges of the same two wheels. Toe can also be defined in terms of an angular relationship of the plane of a wheel (perpendicular to its turning axis) relative to a predetermined longitudinal vehicle axis. When the forwardmost portions of a pair of adjacent wheels are closer together than the rear portions of those wheels, they are commonly referred to as in a "toe-in" condition. When the rear portions of wheels are closer together than the front portions, the condition is commonly referred to as "toe-out."
Camber is a tire wearing angle and also affects handling of the automobile. Caster and SAI do not affect tire wear, but are important for directional control. Toe is a tire wearing angle and also affects the position of the steering wheel required in order to maintain the automobile along a straight line.
Camber, caster and SAI are typically measured using one or more inclinometers which are attached to the wheel. With camber, the inclinometer measures the angle that the spindle makes with horizontal. With caster, the wheel is turned through an arc, and the difference in the camber readings is used to derive the caster value. SAI is measured in a manner similar to caster, except that the inclinometer used for the SAI reading is aligned at 90-degrees to the inclinometer used for reading camber and caster.
The camber, caster and SAI measurements can be made manually with visual inspection and reading, or can be provided on an automatic basis. U.S. Pat. No. 3,913,236 by Butler describes a sensing head which uses gravity sensing variable impedance electrolyte transducers which are connected in a bridge circuit to indicate the angle of the transducer to horizontal.
Over the years, there have been numerous approaches taken to measurement of toe. Initially, manual techniques were used in which the distances between the front edge of the wheels and the rear edges of the wheels were measured and compared. The tie rod turn buckles were adjusted to set the proper amount of toe, and the vehicle was then taken for a test drive to see how the vehicle handled. This approach obviously was time consuming and highly dependent upon the skill of the mechanic.
Various automated toe measurement systems have been developed. One type of system uses head units which are attached to various wheels and which are interconnected by cables or cords. The angles of the head units, and thus the wheels, with respect to the cords are measured by an electromechanical transducer. Examples of this type of measurement device are shown in the Hunter U.S. Pat. No. 4,016,208, the Senften U.S. Pat. No. 4,034,479 and in U.K. Patent Application No. 2,025,064.
Another approach whicH has been used in the past involves optical sensing. Examples of optical systems include the Senften U.S. Pat. No. 3,782,831, the Senften U.S. Pat. No. 3,892,042, the Florer et al U.S. Pat. No. 4,095,902, the Senften U.S. Pat. No. 4,126,943, the Pelta U.S. Pat. No. 4,138,825, the Lill U.S. Pat. No. 4,143,970, the Hunter U.S. Pat. No. 4,302,104 and the Grossman et al U.S. Pat. No. 4,319,838.
There is a continuing need for improved toe measurement systems which are easy to set up, which provide improved accuracy, are lower in cost and are less complex than the prior art mechanical and optical systems.